Formation Of Magnetic Polarons Research Articles

Stability of magnetic polarons in magnetic semiconductor single electron transistors: The effect of Coulomb interaction and external magnetic field

AbstractIn a magnetic single electron transistor (SET), which consists of a magnetic quantum dot (QD) coupled electrically to nonmagnetic source, drain, and gate electrodes, a magnetic polaron (MP) formation may occur, that is, the charge carrier spin may spin‐polarize the magnetic atoms of the QD and simultaneously the carrier becomes more tightly bound to the QD. We have studied theoretically the effect of the Coulomb interaction and magnetic field on the stability of the MPs in ferromagnetic SETs in the Coulomb blockade regime. The calculated results show that the temperature range, where MP is stable can be controlled by the gate voltage of the SET. At temperatures below the Curie temperature the stability decreases with magnetic field, whereas at higher temperatures the opposite is true. The Coulomb repulsion between two charge carriers separates the spin‐up and spin‐down energy levels, which stabilizes the MP further.

Evolution of structural and magnetic properties with varying oxygen content in low-bandwidth manganite Pr0.9Ca0.1MnO3 thin films

The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr1−xCaxMnO3 (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn4+ ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn4+ ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn3+ ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.

Fe doping and magnetic properties of zincblende SiC ceramics

Fe-doped SiC bulk ceramics were fabricated by hot-pressing, and their magnetic and electronic properties were investigated. Si1−xFexC (x ≤ 0.04) samples having a zincblende crystal structure exhibited ferromagnetic hysteresis at room temperature with the saturation magnetization increasing with x. X-ray diffraction measurements revealed the creation of a Fe3Si phase in the samples with its density increasing with x. The samples were found to be p-type semiconductors with a hole concentration (electrical resistivity) of ∼1019 cm−3 (∼100 Ω cm) at room temperature. The observed magnetic properties of the samples are mainly ascribed to the presence of ferromagnetic Fe3Si crystallites. The high carrier concentration of the samples likely implies the existence of acceptors due to individual Fe3+ occupation of the Si sites in the lattice. The randomly distributed Fe3+ ions represent a minor contribution to the magnetization of the samples through the formation of magnetic polarons with the carriers.

ChemInform Abstract: Electric‐Field Controlled Ferromagnetism in MnGe Magnetic Quantum Dots

AbstractReview: 54 refs.

Bound-Magnetic-Polaron Molecule in Diluted Magnetic Semiconductors within Heitler-London Approximation

Magnetic interaction between electron on a shallow impurity and localized magnetic moments has been studied systematically in the last three decades [1–9]. In the first period, the influence of classical fluctuations of magnetization on quantum states of the impurity electron has been analyzed, thus leading to the bound magnetic polaron (BMP) formation. The main result obtained was to show that thermodynamic fluctuations produce a spontaneous spin splitting of the impurity state even without long-range magnetic order appearance, e.g. in a diluted magnetic semiconductor (DMS) [1–4]. A renewed interest was stimulated by the possibility of a ferromagnetic interpolaron interaction [5–7], which leads to parallel alignment of polarization clouds of individual polarons. In this paper recently developed by us [9] a microscopic theory of the bound magnetic polaron molecule (BMPM) in diluted magnetic semiconductors is reformulated in a reduced spin-state space. Namely, we solve the BMPM problem by taking into account the Heitler– London (HL) form of the two-electron wave function in the four-dimensional spin space (comprising one singlet and three triplet states) and include the thermodynamic fluctuations of the magnetization due to the localized 3d spins, in the Gaussian form. In this manner, simplified approaches to BMPM are evaluated and the role of covalency effect is identified and its influence analyzed in a simple workable model (for a full analysis beyond this approximation see Ref. [9]).

Intense laser field effects on p – d exchange interaction in single manganese doped GaAs

We have developed a comprehensive theory about optical control of p – d exchange interaction between spins of hole and Mn2+ in single-manganese doped GaAs material irradiated by a monochromatic, linearly polarized, intense pulsed laser field (PLF) under nonresonant conditions. The p – d exchange interaction leads to formation of magnetic polaron. While the PLF induces a dressed acceptor Coulomb potential, which transforms single center problem into the one with two virtual positively charged centers, resembling hydrogen molecule ion (H2+). The dichotomy of hole wave functions, determined by the laser-intensity, affects strongly the p – d exchange interaction as well as binding energy of magnetic polaron. Increasing the laser intensity reduces the magnetic polaron binding energy. At larger excitation intensity, the magnetic polaron can be completely dissolved.

Features of Magnetic Phase Transitions in EuB6−xCx

The ESR measurements were performed on the single crystals EuB6−xCx with x = 0, 0.02 and 0.07 at X band in the temperature range T = 10 − 300 K. It was observed the magnetic phase separation in spin system of Eu2+ ions The reason of the separation is accompanied by the self-localization of free charge carriers and the formation of magnetic polarons of two types. With decreasing temperatures the 2 phase transitions take place. The first transition is connected with the effect of magnetic ordering of the most intensive subsystem. The second transition is obliged to the delocalization of charge carriers and the system is converted from heterogeneous to homogeneous state.

Zero-phonon emission and magnetic polaron parameters in EuTe

A phonon structure in the photoluminescence of EuTe was discovered, with a well-defined zero-phonon emission line (ZPL). The ZPL redshifts linearly with the intensity of applied magnetic field, indicating spin relaxation of the photoexcited electron, and saturates at a lower magnetic field than the optical absorption bandgap, which is attributed to formation of magnetic polarons. From the difference in these saturation fields, the zero-field polaron binding energy and radius are estimated to be 43 meV and 3.2 (in units of the EuTe lattice parameter), respectively.

Magnetic Polaron Formation Dynamics in Mn2+-Doped Colloidal Nanocrystals up to Room Temperature

Magnetic Polaron Formation Dynamics in Mn2+-Doped Colloidal Nanocrystals up to Room Temperature

Optical Aharonov-Bohm Effect in Type-II (ZnMn)Te/ZnSe Quantum Dots

Photoluminescence for type-II, II―VI Zn(SeTe) quantum dots (QDs) in ZnTe/ZnSe superlattice demonstrate large and persistent oscillations in the energy and intensity indicating the presence of coherently rotating correlated electron―hole states, and the existence of the optical Aharonov-Bohm effect. The magnitude of the effects observed is attributed to the unique geometry of the columnar type-II structures investigated, which creates a natural ring-like geometry. The potential of this geometry to favor the optical AB effect is further demonstrated in similar (ZnMn)Te QD structures. Interestingly, in these magnetic dots the strength of the Aharonov-Bohm coherence is extremely sensitive to the magnetic properties of the dots, which can be enhanced or destroyed according to the spin disorder in the system. This effect is related to the strong exchange interaction between the rotating carrier and the Mn-ions in the dots. In addition, the magnetic nature and type-II band alignment of these QDs results in the formation of robust magnetic polarons, evidence of which persists until > 150 K.

Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism.

Magnetic polarons in ferromagnetic semiconductor single-electron transistors

Magnetic polaron (MP) formation is studied theoretically in a single-electron transistor (SET) consisting of a ferromagnetic semiconductor quantum dot (FSQD) coupled to nonmagnetic source, drain, and gate electrodes. Especially, using Green's-function technique we calculate the effect of the gate-voltage-dependent spin polarization of the charge-carrier spins on the magnetization and conductance of the ferromagnetic semiconductor SET in the Coulomb blockade regime. We apply the Anderson impurity model to the FSQD and the ferromagnetic subsystem inside the FSQD is treated in the mean-field approximation. By minimizing the total free energy of the FSQD we calculate the MP binding energy and the dot magnetization as a function of temperature and the gate voltage. The results show that the ferromagnetic transition temperature of the FSQD increases strongly due to the MP formation, which may contribute to the experimentally observed increase in the Curie temperature in the FSQDs. The calculated results also indicate that due to the MP formation the average magnetization of the FSQD can be controlled by the gate voltage in a wide temperature range. Furthermore, our model predicts that the conductance vs gate-voltage curve, which in nonmagnetic SETs shows a symmetric double peak structure, becomes highly asymmetric due to the MP formation.

Vacancy‐mediated room‐temperature ferromagnetism in Zn1−xMnxO thin films

AbstractThe effect of oxygen vacancies on ferromagnetism (FM) in Zn1−xMnxO thin films grown by radio frequency (RF) sputtering has been investigated. The grown films at different oxygen pressures have been characterized by X‐ray diffraction (XRD), Hall effect, photoluminescence (PL) and vibrating sample magnetometry (VSM). The observed ferromagnetism/diamagnetism by the VSM measurement due to the variation of oxygen vacancies has been confirmed by XRD, PL, and Hall results. The vacancy‐mediated FM has been explained by the formation of magnetic polarons.

Exciton magnetic polaron in CdMnSe/CdMgSe quantum wells

AbstractWe study exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted magnetic semiconductor quantum wells (QWs) using time‐resolved photoluminescence (PL). Magnetic field dependence of the transients allows us to separate the non‐magnetic and magnetic contributions of the exciton localization. We find binding energy of 18 meV and formation time of 500 ps for the EMP. We propose that long polaron formation time is related to auto‐localization process, accompanied with the squeezing of the heavy‐hole envelope wave function. This conclusion is supported by a strong reduction of the exciton radiative lifetime from 600 to 200 ps with increase of magnetic field.

Fine structure of emission lines from charged CdSe/ZnSe/ZnMnSe quantum dots

AbstractPhotoluminescence spectroscopy has been employed to study CdSe/ZnSe/ZnMnSe quantum dots. For most of the dots studied here luminescence comes in three spectrally separated features: neutral exciton (X), biexciton (XX), and charged exciton (XC) states. Spectral properties of X and XX emission are well understood, however, in a marked contrast with previous studies, the observed fine structure of XC can not be explained within a commonly accepted model of a ground state trion luminescence. We find that at zero magnetic field luminescence from the charged state exhibits fine structure that varies gradually between different dots from a single unpolarized line to a quartet with the maximum splitting of 2 meV. Several models including magnetic polaron formation and double charging have been considered, but a plausible explanation can be given only if one considers the influence of a charge trapped in a nearby dot.

Charge transport and magnetization profile at the interface between the correlated metalCaRuO3and the antiferromagnetic insulatorCaMnO3

A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO$_3$ and the antiferromagnetic insulator CaMnO$_3$. The charge carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO$_3$. The small charge transfer across the interface implied by these observations confirms predictions derived from density functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO$_3$, far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.

The effect of oxygen non-stoichiometry and doping with vanadium on the nature of magnetism in titanium dioxide with the anatase structure

Abstract The effect of oxygen non-stoichiometry and doping with vanadium on the electronic structure and magnetic properties of the titanium dioxide with the anatase structure have been studied by employing the tight-binding linear muffin-tin orbitals (TB-LMTO) method in the local spin-density approximation with the account of on-site Coulomb correlations (LSDA+U). It has been shown that the appearance of ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds with 0 ≤ x ≤ 1 / 16 , 0 ≤ y ≤ 1 / 16 is favored by the oxygen non-stoichiometry and location of the exchange-coupled vanadium atoms in the ac and bc planes of the crystal structure. At the vanadium concentration of 0.125 the super-exchange mechanism of the ferromagnetism is essential, with the Curie temperature (340 K) being in good relation to experimental data. The calculations demonstrate that the probability of formation of magnetic polarons is high at the vanadium concentration of 0.0625, but low at the concentration of 0.125. The reduction of magnetic moment with the rise of vanadium concentration corresponds to the general trend revealed for dilute magnetic semiconductors. It was established that the mechanisms of the high-temperature ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds are different and depend on oxygen non-stoichiometry and vanadium concentrations.

Effect of chromium disorder on the thermoelectric properties of Layered-antiferromagnet CuCrS2

Layered-antiferromagnetic compound CuCrS2 has been prepared by different methods. The analysis of X-ray diffraction patterns of different samples gave significant amount of vacancy-disorder of Cr-atoms within the layers. Extended period of sintering above 900 °C increases the transfer of Cr-atoms to the interstitial sites between the layers. This disorder has marginal effect on the Antiferromagnetic properties. The electrical conductivity is increased and the thermoelectric power remains positive and quite high between 150–400 μV/K in the paramagnetic state around room temperature with increase in disorder in different samples. We interpret the temperature dependence of electrical resistivity and thermoelectric power due to the localization of carriers by interstitial defects and the formation of magnetic polarons in the paramagnetic phase of CuCrS2.

Formation of magnetic polarons in lightly Ca doped LaCoO3

We performed high field electron spin resonance, nuclear magnetic resonance and static magnetization measurements on a powder sample of lightly hole-doped La1−xCaxCoO3, x ~ 0.002 in order to study the influence of the size of the substitution ion on the formation of the hole induced spin-state polaron. Previous works [1, 2] showed that doping of LaCoO3 with Sr in very small concentrations (x ~ 0.002) yields the formation of a magnetic polaron with a big spin value and large spin orbital coupling. The Ca2+ ion, in contrast to the Sr2+ion, has almost the same ionic radius as the La3+ ion. Therefore, the substitution of Ca for La provides mainly a hole to the system without creation of a sizeable crystal field distortion around the substituted Ca ion. The data obtained on La0.998Ca0.002CoO3 provide experimental evidence that the introduced hole indeed plays the main role in the formation of the spin-state polaron. Accompanying crystal field distortions seem to play a minor role, e.g. influencing the fine splitting of the spin polaron energy levels and the contribution of the spin orbital coupling.

Mixed-valence manganites

Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...